U.S. Pat. No. 3,816,840 describes an electrostatic printing process and printer in which a dielectric image-receiving material is fed between a first and a second electrode which are disposed a short distance apart. One of the electrodes is covered with a layer of magnetically attractable electrically conductive toner powder. Voltage pulses are applied between the electrodes so that toner powder is deposited on the image-receiving material in the form of an information pattern. A disadvantage of this process is that only dielectric image-receiving material can be used, thereby restricting the choice of usable image-receiving materials.
U.S. Pat. No. 3,946,402 describes an electrostatic printer comprising a rotatable drum provided with a dielectric layer on which a uniform layer of electrically conductive magnetically attractable toner powder is applied. A magnetic roller is disposed in an image-forming zone near the drum surface covered with toner powder and has a stationary nonmagnetic sleeve and a rotatable magnet system mounted inside the sleeve. A large number of magnetic electrodes in the form of rods each connected to a voltage supply are disposed axially on the sleeve of this magnetic roller. When the electrodes are not energized, toner powder is attracted from the drum surface to the magnetic roller. No toner powder is attracted when the electrodes are energized. By energizing the electrodes pulse-wise according to an information pattern, a toner image corresponding to the information pattern is formed on the drum and can then be transferred to a receiving support.
Since the electrodes are conductive they must be insulated from one another. A disadvantage of this device is that the conductive toner powder can short-circuit some electrodes and thus disturb the image formation. Another disadvantage is that it is a very complex and expensive matter to construct the row of fine magnetic electrodes in rod form used in this device.
Japanese Application No. 59-224368 shows an image-forming element with a rectangular grid of electrodes. The grid is made from two groups of parallel electrodes with each group being perpendicular to the other. There are no electrical connections between the two groups of electrodes forming the grid. Moreover, both groups of electrodes are attached to a power source and both groups must be activated to generate a sufficient potential to attract toner particles at the point where the overlying electrodes cross.
Japanese Application No. 59-224369 shows an image-forming element comprising an endless support with an insulating layer and a multiplicity of electrodes embedded in the insulating layer. Preferably, the electrodes form rings around the circumference of the support and are selectively electrified from within the support. Only one set of electrodes is provided in this device.
U.S. Pat. No. 3,739,087 shows a recording element consisting of a cylindrical wall of insulating material with a multiplicity of electrodes extending through the cylindrical wall. One disadvantage of this arrangement is that if conductive toner is used, it can form a bridge between the ends of the electrodes and short-circuit some of them, thereby disturbing the image formation.
Accordingly there is a need for an image-forming element for an electrostatic printer which obviates the above-mentioned disadvantages.